How to create a 3D LSystem?
Step 1)  Make a plane then enter Edit mode.

Step 2)  Use Alt + m to merge the points of the plane to a single point. 

Step 3)  Use Ctrl + mouse-click to extrude points out.(Which mouse button depends on whether you have right or left click as default... I think...)

Step 4)  Add a sphere to a different layer

Step 5)  Add a particle system to the lines using the settings that I've circled in the following image.  The image also shows what an OpenGL render will look like using the default settings.  

(remember to right-click any images that are difficult to see and choose -> 'View Image' to see the larger version)



Now add a skin modifier and using default settings, enter Edit mode and select all the points.  Now scale all the points using Ctrl + A, so far as I know this is the only tool that uses Ctrl + A.  Normally scaling is done with s or by normals using alt+s.

That's pretty much it except that I did apply the Particle System in the modifier tab and since there may be overlapping spheres at some of the 'tree' vertices, you may want to join all the spheres then enter Edit mode, select everything using a and then use the Remove Doubles tool to clear up any overlapping vertices.